They were being captured so Du Ze had no choice but to summon the sword for defense. Also, one was unconscious while the other has the build of a child. They couldn't stop the demon city lord from chopping Du Ze's hand and eating it.
If that was true how did he stop the sword with his hand
Well the reason is a spoiler, I'm not sure if you'd like one.
Warning
*spoilers*!!!
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He's currently awakening his demon form for the first time.
reading the novel THEN waiting for this actually good for your soul XD.
Although the manhua didn't stick to the novel, I still appreciate it's existence ⊂(´･◡･⊂ )∘˚˳°
Why would he spawn in the sword? Also why tf did he watch the guy chop off his hand??? When he could stop it that pissed me off